The present invention relates to rotary drilling and, more particularly, to a directional drilling technique for providing deviated wellbores at significantly greater inclinations and/or over horizontal distances substantially greater than that currently being achieved by conventional directional drilling practices. The success of such directional drilling should benefit mainly offshore drilling projects as platform costs are a major factor in most offshore production operations. Wellbores with large inclination or horizontal distance offer significant potential for (1) developing offshore reservoirs not otherwise considered to be economical, (2) tapping sections of reservoirs presently considered beyond economical or technological reach, (3) accelerating production by longer intervals in the producing formation due to the high angle holes, (4) requiring fewer platforms to develop large reservoirs, (5) providing an alternative for some subsea completions, and (6) drilling under shipping fairways or to other areas presently unreachable.
A number of problems are presented by high angle directional drilling. In greater particularity, hole inclinations of 60.degree. or greater, combined with long sections of hole or complex wellbore profiles present significant problems which need to be overcome. The force of gravity, coefficients of friction, and mud particle settling are the major physical phenomena of concern.
As inclination increases, the available weight from gravity to move the pipe or wireline string down the hole decreases as the cosine of the inclination angle, and the weight lying against the low side of the hole increases as the sine of the inclination angle. The force resisting the movement of the drill string is the product of the apparent coefficient of friction and the sum of the forces pressing the string against the wall. At an apparent coefficient of friction of approximately 0.58 for a common water base mud, drill strings tend to slide into the hole at inclination angles up to approximately 60.degree.. At higher inclination angles, the drill strings will not lower from the force of gravity alone, and must be mechanically pushed or pulled, or alternatively, the coefficients of friction can be reduced.
In each of U.S. Pat. Nos. 2,500,267 to Zublin; 3,260,318 to Neilson, et al.; and 3,667,556 to Henderson, there is described rotary drilling apparatus in which a drill string is rotated to turn a downhole drill bit during conventional drilling operations and in which the drill string is held stationary while the drill bit is rotated about an axis inclined from that of the drill string during the deviating of the direction of the wellbore.